Radioactivity and Ionizing Radiation Flashcards
Radioactive decay
- what are the two types of radionuclide
- describe decay rate
- which laws are conserved during radioactive decay
Radioactive atoms are those with unstable nuclei (radionuclides) which will spontaneously decay by emitting a particle or a quantum of electromagnetic radiation.
-Their decay may lead to the formation of a stable or unstable daughter nuclei.
There are two basic types of radionuclides:
- Natural: occur in nature. Light natural radionuclides: atomic # less than 75 amu, do not form decay series, resulting nuclei are stable. Heavy natural radionuclides: atomic # (Z) equal or more than 93 amu; form decay series in which parent nuclei give rise to daughter nuclei
- Artificial: are produced artificially in atomic reactors or accelerators
Decay Rate: Number of nuclei at time t: Where N is the number of nuclei at time t, e is euler’s # for natural logarithms λ is the disintegration or decay constant: it represents the relative rate of decay - unit is s-1 *The # of radioactive nuclei decreases exponentially with time *The value of the ln N decreases linearly with time Activity: the # of nuclei that decay in 1s A = λN -activity can be used to estimate the decay rate -activity decreases exponentially with time -unit of activity is the Becquerel (Bq) -a radioactive sample will have an activity of 1 Bq if the number of nuclei that decay in 1s is 1.
- Basic laws conserved during radioactive decay:
- The law of conservation of electric charge: the sum of charges of the nucleus stays constant – therefore if a negative charge is emitted from the nucleus during decay, it gains a positive charge
- The law of conservation of nucleons count: the number of nucleus also stays constant – therefore the # of nucleus in the parent generation equals the sum of the # of nucleons in the daughter generation and the emitted nucleons
- The law of conservation of momentum: the sum of momentum of daughter generation and emitted particles equals 0. Therefore they move in opposite direction.
- The law of conservation of energy: the amount of energy equals the sum of the emitted particles and daughter particles
α, β and γ Radiation
- what is an alpha particle?
- emission of an alpha particle effects parent atom how?
- what happens if the energy of the α particle emitted is lower than the transmutation energy
- what are the thee types of B-decay
- Discuss gamma radiation
α decay: Decay of heavy radionuclides results in emission of an α particle.
- α particle:* composed of 2 protons + 2 neutrons. It is the nucleus of a Helium atom.
- has a charge of +2
- α particle are absorbed by material very easily and do not penetrate shielding material very far.
- emission of an α particle results in a daughter nucleus with an atomic # of 2 less than the parent and an atomic mass of 4 less than the parent nucleus. The daughter nucleus is to the left of the parent on the perdiodic table by 2 positions.
The parent nucleus changes according to the scheme :
-if the energy of the α particle emitted is lower than the transmutation energy, then the daughter nucleus that is formed is in the excited state and can fall to the ground state by emission of gamma radiation
β decay : is known as an isobaric transmutation of the nucleus since nucleon number is conserved.
There are 3 types of β decay:
1. β– Decay: Emisson of Electron and its antineutrino: A neutron decays into a a proton and β- particle
Mass number is conserved and atomic number increases by 1 (new proton)
* the electrons produced possess a continuous energy spectrum
- β+ Decay: Emission of positron: A proton decays into a positron and an neutron
Mass # is conserved and atomic # decreases by 1 (a proton has decayed)
* the positron energy spectrum is continuous
*the position of the daughter nucleus ist shifted to the left by 1 position in perspective of the parent generation
-
Electron Capture: occurs when unstable radionuclides capture an electron from the inner K shell. The electron combines with a proton to form a neutron.
- The mass number is once again concerved but atomic number decreases by 1
*characteristic electromagnetic radiation is emitted
*Excited nuclei are formed in all three types of B decay and immediate transition into the ground state occurs with simultaneous emission of gamma radiation
Gamma radiation:
Gamma decay- This radiation is represented by gamma rays or high energy photons emitted by the nuclei of radioactive elements.
- Since gamma rays do not carry charge or mass, both the mass number and atomic number is conserved in the nucleus.
- The cobalt gun is used as a source of radiation used in radiotherapy. Uses radioactive cobalt -60
- The known intensity of y-radiation decreases slowly with time since the half-life of this radionuclide is about 5 years.
- It can used in therapy of malignant tumors by using ionising radiation.
Radioactive Equilibrium
In a series of radioactive decay, an equilibrium will be reached at which time identical #s of parent + daughter nuclei decay per unit time.
-If T1 (half life of parent) << T2 (half life of daughter), then the activity of the parent decreases due to its short half-life and and the activity of the daughter increases initially and then decreases due to its own decay rate
*no equilibrium
- If T1>T2, then the activity of the parent decreases according to its decay rate.
- and since the half life of the daughter is short, the # of daughter nuclei increases up to a maximum and then their activity decreases proportionally to the parent
*transitional equilibrium
- If T1>>T2, (the half life of the parent is so long that it does not decay during measurement) then the activity of the daughter will rise until it reaches that of the parent. Thus the # of nuclei formed will equal the number that is decaying
- At this point the parent and daughter nuclei are in a state of permanent radioactive equilibrium.
- Thus the ratio of the number of parent and daughter nuclei equals the ratio of their half lives.
Physical, biological and effective half-life
- unit of half life is time (days, secs, hrs, years, whatever is appropriate)
- Physical Half-life:* (Tf) The time it takes for ½ of the radioactive nuclei in a sample at zero time to decay.
- Biological Half-life:* (Tb)
The time required for half the quantity of a drug or other substance deposited into a living organism to be metabolized or eliminated by normal biological processes.
Effective Half-life: The time required for the radioactivity of material administered or deposited into an organism to be reduced to half its initial value by a combination of biological elimination processes and radioactive decay.
The relative disappearance rate λef is the sum of the excretion rate and the decay rate.
Absorption of Gamma (γ) radiation
- how does radiation interact with matter
- photoelectric effect - probability of absorption
- compton scattering
- formation of electron-positron pairs
- half-thickness
- half-layer
*When passing through absorbers, radiation loses energy. Energy loss depends on both the type of radiation and absorber
Range:the distance that radiation can pass through an absorber
Radiation can interact with matter in several ways: ionization, excitation, scattering, production of brehmsstrahlung, and nuclear reactions
- ionization is the most significant source of energy loss for charged particles
- Gamma radiation is high energy electromagnectic radiation that is highly penetrating.
- Only dense materials such as lead are capable of absorbing such high energy radiation.
- The attenuation coefficient for gamma radiation is the sum of the three attenuation coefficients for the photoeffect, compton scattering, and formation of electron-positron pairs.
- The photoelectric effect*: a photon will transfer all of its energy to an electron in the shell of an atom with which it is interacting. A part of this energy will contribute to the ionization energy required to completely remove an electron from an atom. The remainder of the energy will be converted to the kinetic energy of the electron as it leaves the atom.
Probability of absorption due to the photoelectric effect:
- Thus the photoelectric effect is most probable at low energies and in heavy absorbers.
- This type of absorption is most probable in tissues with high Z – for ex: bone
- Compton scattering-* occurs at high photon energies
- In this type of interaction, the photon interacts with a free electron in the absorber.
- Part of the photon’s energy is transferred to the electron and the electron and photon move away from each other in a scattered direction. The resulting photon has a lower energy.
- The energy of the resulting photon is dependent on the scattering angle
- The highest decrease is expected for backscattering where the angle is 180 degrees.
- This process may be repeated several times until the photon donates the last bit of its energy via the photoelectric effect.
- Formation of electron-positron pairs:* probability of this type of absorption is proportional to the energy of the photon.
- -*also more probable in absorbers with higher atomic numbers
- -*At high energies, the photon will disappear and its energy will be converted to the rest mass of the electron-positron pair and their kinetic energy.
- Half-thickness:* the thickness of the absorber which reduces the intensity of incident radiation by 50%
- Half-layer:* the mass per unit area of absorber (kg/m2) with density p that reduces the intensity of incident radiation by 50%
Absorption of Alpha and Beta radiation
- Bremsstrahlung
Alpha particles- Due to their relatively large mass and electric charge, the ionization losses of energy are high and several 1000s of ion pairs may be formed during the absorption of one alpha particle.
-Energy losses due to ionization and excitation are approx 50/50.
-The range of penetration is very small. i.e. with an energy of 10 MeV, penetration is about 10cm in air and several µm in soft tissue or water. → therefore apha particles can have a negative biological effect when passing through tissue bc all energy is concentrated into several micrometers of tissue.
Beta-
- Ionization and excitation represents the highest energy losses of electrons during their passage through an absorber.
- Their specific linear ionization is lower than alpha because of the lower mass to charge ratio.
- In addition to ionization and excitation, Bremsstrahlung can also be produced.
- But the energy losses due to bremsstrahlng are relatively low and more important at high energies
-Bremsstrahlung is high energy quanta of electromagnetic radiation that is produced when accelerated electrons are stopped in the electrical field of an atomic nucleus.
- The intensity of Bremsstrahlung is proportional to the atomic number of the absorber and the electron’s energy.
- Bremsstrahlung radiation produces a continuous energy spectrum
The intensity of a beta beam decreases according to:
*in soft tissue the range of beta-absorbtion lays in the mm range for most beta-emitters
Selective and integral detection of Gamma radiation
define Energy resolving power (R) of a scintillation head
Because the amplitude spectrum produced by scintillators possess continuous character due to the presence of compton scattering, using an amplitude discriminator, two modes of detection can be performed:
- Integral detection*: all pulses with amplitudes greater than a set level are counted, The total # of pulses is proportional to the area under the curve of number of pulses plotted as a function of energy.
- Selective detection*- at photopeak, only pulses with amplitude between the lower level and upper discriminatoion level are registered;
- advantage is good spatial resolution of head and decreased detection of cosmic rays and scattered radiation.
Energy resolving power (R) of a scintillation head is defined by the width of the photopeak measured at half its height expressed as a function the mean energy in percent
Principles of detection of ionising radiation
what are the three types of detectors used for ionizing radiation?
Detection efficiency
-Detection of various forms of radiation is based on interaction of the radiation with the sensitive part of the detector.
Detectors convert radiation energy into other forms of energy that can be registered by other devices.
-After a detector absorbs radiation, it generates electrical pulses. The pulses produced by the detector are amplified, formed in shape, measured, and individually registered or their mean count rate is calculated by other parts of the radiation measurement device. The pulses are registered by the counter.
Detection efficiency: the ratio of the number of particles registered by the detector to those that pass through the detector.
-Multiple types of detectors are available and their function is based on the various interaction radiation may have: ionization, excitation, chemical, thermal, or photographic.
***The three types of detectors used for ionizing radiation are the ionization chamber, the geiger-muller counter, and the scintillation counter
what are the three detectors of ionising radiation?
The three types of detectors for ionizing radiation are the ionization chamber, the Geiger-muller counter, and the Scintillation counter
Scintillation detector
- detects what type of radiation
- three parts
- efficiency
- scintillation head
- The scintillation detector is used for detection of gamma radiation
- It consists of 3 parts
1) Scintillator- converts radiation to luminescence (crystals of sodium iodide NaI are frequently used as the scintillator)
2) Photomultiplier - detects scintillations
3) electronic parts fore registration of the signals
- Atoms of the scintillator are excited upon absorption of gamma radiation
- Photons of visible or ultraviolet light are then emitted during deexitation of the electrons in the atoms
- The resulting luminescent photons impact the photocathode of the photomultiplier and cause emission of electrons via the photoelectric effect.
- The photomultiplier consists of 8-14 dynodes which are electrodes with subsequently increasing voltage. The 1st dynode has the lowest voltage. It ejects electrons which then are accelerated to the next dynode and cause ejection of electrons there
- Thus many electrons are formed due to the impact of the first. This leads to great amplification (105 – 107)
- The avalanche of electrons come to final dynode and creates a pulse of voltage
- The pulses are amplified and measured.
Detection efficiency = 30 – 50% for gamma radiation
Scintillation head – the scintillator and photomultiplier (and preamplifier) are located within the same container to prevent light from entering from the outside
Geiger-Muller tube
- used for what?
- structure
- method
- detection efficiency
- Used for the detection of beta radiation
- The Müller tube consists of metallic cylinder which represents the cathode and an axially mounted tungsten wire which represents the anode.
- The tube is filled with an inert but ionizable gas (mostly argon and some other polyatomic gas)
- A high voltage is set up between the electrodes
- When radiation enters the tube, the gaseous atoms are ionized.
- Due to the high voltage difference between the electrodes, the newly formed ions are accelerated to high kinetic energies and may cause more ionization in other gaseous atoms
- Thus an electron avalanche is created from only one ion pair.
- This produces a pulse in the electrical current within the tube that can be registered by the counter part of the measuring device.
When count rate is plotted as a function of voltage, a plateau is formed at higher voltages. At these points, the count rate is independent of the applied voltage
Detection efficiency = ratio of number of registered particles/ photons passing through detector
Accelerators of particles
- purpose?
- method?
- medical practicalities?
- two types?
- accelerators of negatively charged particles?
- Instrument that yields charged particles with high kinetic energy
- These high energy particles are then used for bombardment of target nuclei in order to induce nuclear reactions or high energy bremsstrahlung
- Accelerators are typically used to accelerate positively charged particles.
+ charged particles such as alpha particles, protons, and deuterons are good sources of ionizing radiation however they do not have sufficient energies. Thus using a potential differences to accelerate the particles increases their kinetic energy and allows them to be used for practical purposes
- In medicine, they are used in the production of short lived radio nucleotides (betatron) or the production of High Energy Bremsstrahlung to destroy malignant tumors
- There are two types of acceleratorss –Linear or Circular
A) Linear accelerators:
-Particles are accelerated when passing through the the straight or high frequency accelerating tube
Electrostatic: power source for the voltage difference is the Van de Graaf generator
High frequency linear accelerator => power source is the klystron generator
- 20-40 kV accelerator.
- Ions pass through gaps between metal cylinders of alternating polarity (the charge on the first cylinder must be opposite of ion)
- There is stepwise increase in the lengths of the cylinders since velocity of accelerated ions in each cylinder is higher than the previous one.
- the target atoms are located at the end of the last cylinder
- acceleration process takes place in a vacuum.
b) Circular Accelerators: - Cyclotron:* used to accelerate heavy particles (protons, deuterons, alpha particles)
- -consiste of 2 parts or *Duants connected to high freq alternating voltage which creates an alternating electrical field in the space between the duants
- The source of the particles is located between the two duants
- The particles are attracted to the oppositely charged daunt and simultaneously accelerated upon entering the magnetic field.
- The magnetic field causes the particle to move in a circular motion
- The polarity of the duants changes repeatedly so that the particle is attracted to one duant and then to the other.
- the radius of the particles path increases as does its velocity and energy (inc. Volt & Elec)
Particles leave as a continuous flux
*Used in medicine in the production of short lived radio nucleotides for diagnostic purposes.
c) Accelerators of Negatively charged particles:
Betatron- device for the acceleration of e-
- The accelertion takes place in an evacuated glass ring
- The ring is situated at the poles of an electromagnet and pulsed jets of e- are delivered into the ring
- A periodically changing magnetic induction
results in variable magnetic flux
which produces electromotoric force proportional to the variation of magnetic flux.
-The EMF accelerates e- during the 1st period when magnetic induction increases -Therefore the Betatron=pulse accelerater.
The accelerated e- move w/ relativistic velocity
E= expressed in eV, r=radius and Bm teslas
-The accelerated electrons then hit the target to produce high energy Bremsstrahlung
*** Because neutrons do not have a charge they cannot be accelerated ***
Ionisation chamber
- structure
- higher operating voltage yields what?
- used as what?
This is basically a capacitor (seperates and stores charge with a potential difference between the parallel plates) with 2 electrodes filled with air.
- Ion pairs (electrons and positively charged ions) are formed in the chamber though interaction with ionizing radiation and are then attracted to the electrodes.
- Therefore, an electric current flows through the chamber and can be registered when exposed to radiation
- The higher the operating voltage, the higher the accelerating force, the greater the intensity of the electrical current flowing through the chamber
- The produced ions can disappear at the electrodes, chamber walls or by recombination.
- Ions with charge e move in electric field due to force F=eE where E is intensity of electric field.
** Ionisation chambers are used as pocket personal dosimeters for evaluation of exposure to radiation.
Methods of personal dosimetry
- function based on which 3 interactions of radiation with matter?
- disadvantages?
Personal dosimeters are used to determine the amount of ionizing radiation that people who work with radionuclides
Their function is based on 3 different interactions of radiation with matter
(1) Film-Dosimetry (effect on photographic emulsion) a photographic emulsion that is sensitive to ionizing radiation is exposed to ionizing radiation and after its development the dose is measured by the amount of blackening on the film.
-not accurate below 5 μC/kg
(2) Thermoluminesence Dosimetry (based on the excitation effect of radiation)- The electrons of lithium flouride crystals are excited to higher energy levels upon exposure to radiation. They remain in the excited state until temp is increased (100 ºC) and then deexitation occurs with emission of visible light. The intensity of the light is measured by a photomultiplier and it is proportional to the dose absorbed by the crystal.
- good for 3mnths, work good at room temp and humidity
(3) Pocket Dosimetry – (based on the ionizing effect of radiation)- utilizes a small ionization chamber filled with air and charge. The ionization effect creates ion pairs and electrical charge decreases thus measuring the dose
disadvantage: sensitive to humidity
Units of exposition and absorbed dose of radiation
- Emission
- equation for radioactivity
- Exposure
- Absorbed dose
- Dose rate
- Dose equivalent
The basic quantity which describes a radiation source is emission
-Emission is the number of particles ( quanta) emitted by the source in one unit of time, s-1
Emission = # of quanta divided by second
- If the source is radionuclide, its emission is related to its activity (A).
- Activity: the # of nuclei that decay in 1s
A = λN
- activity can be used to estimate the decay rate
- activity decreases exponentially with time
- unit of activity is the Becquerel (Bq)
- a radioactive sample will have an activity of 1 Bq if the number of nuclei that decay in 1s is 1.
est s-1imated decays divided by one second unit is Bq which is becqurel
Bq used to be known as curie( ci) where 1 ci = 3.7 x 1010 bq..
- Based on the decay curve, it is easy to estimate the activity of the pharmaceutical preparation at the time of its application to the patient.
- Because activity decreases exponentially with time, a larger volume of the radionuclide preparation must be administered to ensure the proper dose
.
Mass , M of radionuclide = 2.073 x 10-15 A ( Mbq) Tf
M= mass expressed in micrograms A =nucleon # Mbq= activity expressed in mbq, and Tf = half life in days.
Exposure: is a characteristic of x-ray and gamma radiation and it is based on ionization effects and estimates the ionizing power of radiation in the air.
Exposure X = ratio of electric charge, ∆Q, of ions created by complete absorption of particles ( electrons and positrons) formed by interaction of X- of Y- rays in vol of air with mass ∆m.
X= delta Q / delta m
Unit of exposure is Coulombs/kg
The formerly applied unit was the roentgen
Exposure Rate dX/dt is the change in exposure over change in time.
Its unit is A/kg and dimension is m2s-2A
- Absorbed dose D* = ratio of mean energy of ionizing radiation (∆E) absorbed to its mass. Unit is gy ( grey) 1gy = J/kg -1.(1 W.kg^-1)
- formally applied unit was the rad
- Dose rate* = mean increase of dose, (∆D), in time interval (t)
dD/dt = ∆D/∆t unit is W.Kg -1.
Dose equivalent H is applied for the evaluation of the radiation effect on living organisms
H = D*Q*N
Q=quality factor of the type of radiation (ex. Q=1 for X- or Gamma-radiation)
Unit = Sievert (S)
